Help...nasturtiums!!

Have made some hanging baskets up, the first I have done for about 10yrs! I put some trailing nasturtiums in, and sadly the leaves are all turning yellow! Any ideas why this is happening? Overwatering? compost too rich??I know they can thrive on/in poor soil...maybe the slow release fertilizer pellets in the baskets are too strong for them?
Any solutions greatly received. Oh, they are still hanging up in the greenhouse at present, establishing.[This is an old greenhouse which has no ventilation, except when I have the door open] I hope to put them out side this week..the forecast for up here looks a bit warmer at last!